Technics SU 600 Repair – Minor Faults and Section Function

Technics SU600 Specifications:
Continuous Power Output per Channel:
30 W :  20 Hz – 20 kHz at 8 Ω, (Total Harmonic Distortion 0,07%)
50 W :  1khz DIN at 8 Ω
Input Sensitivity/Impedance:
Phono MM :  2,5 mV/47 k Ω
Tuner, CD, Aux, Tape :  150 mV/22 k Ω
Frequency Response:
Phono :  RIAA ±0,8 dB
Tuner, CD, Aux, Tape :  5 Hz -70 kHz (-3 dB)
Signal-to-Noise Ratio:
Phono :  71 dB (72 dB IHF-A)
Tuner, CD, Aux, Tape :  90 dB (98 dB IHF-A)
Power Supply/ Consumption:  240 V 50/60 Hz ; 250 W
Dimensions (W x H x D):  430 97 x 240 mm
Weight:  5,0 kg

The minor faults are:

The rectified DC rails at the 4700uF caps are slightly asymmetrical around +/- 47V DC and +/- 15V DC:

I’ve got loads of leads and connectors ordered so can do some waveform testing soon on the stereo signal mismatch, and get to hear some music through this and do a power measurement when the 100W, 8ohm resistors arrive from China…

The full wave DC rectification is spot on at the diodes around 100Vpp or 50V peak per rail:

Note the subtle voltage difference – 43.3V and 43.1V lines – by a small difference in the 2W resistor values of 330 and 390 ohms. These rails power pins 2 and 3 of the SV13102A power amp. The audio ins are pins 11/13, with 12 at ground:

As these resistor values have actually drifted with age also, there is a volt or 2  difference between the positive and negative rails, but the +/-15V.3V are clamped by the stacked Zener diodes:

Part #: MA4075-L
Part Category: Diodes
Manufacturer: Panasonic
Description: Zener Diode, 7.3V V(Z), 2.48%, 0.4W, Silicon, Unidirectional, DO-34
Part #: MA4068
Part Category: Diodes
Manufacturer: Panasonic
Description: Zener Diode, 6.8V V(Z), 5.88%, 0.37W, Silicon, Unidirectional, DO-34

These Zeners don’t quite add up to +/-15.1V but 14.1V from their specs…?

The +/-15V rails power the AN6552F phono stage op amp:

and the M5218 tone amp:

Technics SU600 Specifications:
Continuous Power Output per Channel:
30 W :  20 Hz – 20 kHz at 8 Ω, (Total Harmonic Distortion 0,07%)
50 W :  1khz DIN at 8 Ω
Input Sensitivity/Impedance:
Phono MM :  2,5 mV/47 k Ω
Tuner, CD, Aux, Tape :  150 mV/22 k Ω
Frequency Response:
Phono :  RIAA ±0,8 dB
Tuner, CD, Aux, Tape :  5 Hz -70 kHz (-3 dB)
Signal-to-Noise Ratio:
Phono :  71 dB (72 dB IHF-A)
Tuner, CD, Aux, Tape :  90 dB (98 dB IHF-A)
Power Supply/ Consumption:  240 V 50/60 Hz ; 250 W
Dimensions (W x H x D):  430 97 x 240 mm
Weight:  5,0 kg

The minor faults are:

The rectified DC rails at the 4700uF caps are slightly asymmetrical around +/- 47V DC and +/- 15V DC:

I’ve got loads of leads and connectors ordered so can do some waveform testing soon on the stereo signal mismatch, and get to hear some music through this and do a power measurement when the 100W, 8ohm resistors arrive from China…

The full wave DC rectification is spot on at the diodes around 100Vpp or 50V peak per rail:

Note the subtle voltage difference – 43.3V and 43.1V lines – by a small difference in the 2W resistor values of 330 and 390 ohms. These rails power pins 2 and 3 of the SV13102A power amp. The audio ins are pins 11/13, with 12 at ground:

As these resistor values have actually drifted with age also, there is a volt or 2  difference between the positive and negative rails, but the +/-15V.3V are clamped by the stacked Zener diodes:

Part #: MA4075-L
Part Category: Diodes
Manufacturer: Panasonic
Description: Zener Diode, 7.3V V(Z), 2.48%, 0.4W, Silicon, Unidirectional, DO-34
Part #: MA4068
Part Category: Diodes
Manufacturer: Panasonic
Description: Zener Diode, 6.8V V(Z), 5.88%, 0.37W, Silicon, Unidirectional, DO-34

These Zeners don’t quite add up to +/-15.1V but 14.1V from their specs…?

The +/-15V rails power the AN6552F phono stage op amp:

and the M5218 tone amp:

The LED Power and Safety on lights work off the 12V line (8.8 RMS winding).  This is rectified by the single diode D701 and capacitor C701:

At power on the first Power LED lights immediately, but the Safety LED is only connected to ground once the power amp diagnostic switches pin 7 high (0-4V) at the gate of Q602 FET, which turns on the base of Q603 NPN to allow the Safety LED a path to ground:

The LED Power and Safety on lights work off the 12V line (8.8 RMS winding).  This is rectified by the single diode D701 and capacitor C701:

At power on the first Power LED lights immediately, but the Safety LED is only connected to ground once the power amp diagnostic switches pin 7 high (0-4V) at the gate of Q602 FET, which turns on the base of Q603 NPN to allow the Safety LED a path to ground: